The present invention relates to a distance amplitude compensation system and more particularly to a system for providing a reference signal that has an amplitude that varies as a function of time.
The use of ultrasonic pulse echo techniques to test workpieces has been long known. Such techniques typically provide for the periodic generation of high frequency electric pulses and applying them to a piezoelectric element which transforms the electric vibrations into mechanical vibrations which are then transmitted into the object under test. The transmitted pulses are reflected from the rear boundary of the workpiece as well as from any internal discontinuities in the workpiece such as cracks, inclusions and flaws. The presence of such defects can therefore be determined by detecting the reception of pulses reflected by the piezoelectric element which transforms the detected mechanical pulse vibrations into electric voltage signals. The electric signals may be processed to be viewed on a cathode ray tube or other similar utilization device to determine flaws in the workpiece.
The magnitude of signals detected which indicate flaws would be ordinarily a function of the size of the defects. However, the signal is also a function of the distance of the defect in the workpiece below the entering surface of the object. It is therefore necessary in order to obtain visual and automatic interpretation of flaw size to recognize the continuous change in echo signal from a given size flaw as a function of its distance from the entering surface of the workpiece under test. This change in echo signal is non-linear and usually bidirectional. Thus, the response signal increases for a short distance in the workpiece (near field zone), reaches a peak (near field limit), and then continues to decrease throughout the remainder of the test piece, (far field).
This problem was recognized by Weighart as documented in his U.S. Pat. No. 3,033,029. This prior art patent describes a gain control system with complex wave shapes for the control voltage and discusses the result of the near field and far field effects of the beam geometry as well as the exponential attenuation with depth in the material. As shown in FIG. 1 of the Weighart patent, not only does the quantity of returned energy vary with distance within the test piece, but it varies nonlinearly as a complex function of distance (or time). The lower amplitude response for short distances is due to the near field effect. As illustrated in the Weighart patent, the signals first increase with distance below the surface and then decrease with distance below the surface.
Various prior art devices have sought to compensate for this effect by the use of distance amplitude compensation techniques, such as shown in Weighart, which change the gain of the receiver. Such devices have typically been referred to as distance amplitude compensation systems.
These conventional devices have sought to provide distance amplitude compensation by changing the gain of the receiver amplifier rapidly with each sweep trace, e.g., several dB in a few microseconds. Such devices are disclosed in Radar Handbook, by Skolnik, Ed., McGraw-Hill, New York, N.Y. There are several limitations and disadvantages to conventional distance amplitude compensation systems using the time-varied gain method. Very fast gain changes cannot be made easily because transients are introduced in the amplifier which produce false signals. The distance amplitude compensation function cannot be abruptly terminated at the start of the back reflection, which is ordinarily very large compared to flaw signals. This aggravates the problem of displaying the desired back echo on-screen when applying "back-echo gain" control. The linearity and/or dynamic range of the amplifier may be adversely affected by the distance amplitude compensation control using this time-varied-gain method.
Another problem with typical prior art systems relates to the set-up of such devices. Ordinarily, the operator must be provided with a distance amplitude response function curve which was previously obtained experimentally. Alternatively, the operator may be provided with a set of distance amplitude test blocks and be required to establish his own curve. If the effect of gain control function is not displayed on the CRT, the operator must use trial and error techniques to establish the echo signals from various depth blocks, an almost impossible task. In general, only one echo at a time from each block can be displayed. Even if the distance amplitude compensation control voltage can be displayed on the CRT, various trial and error adjustments of the several distance amplitude compensation controls must be made to compensate for the near field slope, far field slope, amplitude, and delay so that the desired gain effect may be obtained.
It can be shown that for the usual distance amplitude compensation method of receiver gain control, the voltage needed at the amplifier is a non-linear function of the distance amplitude response curve. If curve matching is used to set up the distance amplitude compensation system, a rather elaborate and very precise electronic method must be employed to present the inverse of the distance amplitude compensation control signal in both shape and absolute level referenced against the echo amplitude signals to be corrected. For curve matching, the distance amplitude compensation waveshape must be displayed on an alternate sweep trace in order not to be superimposed on the regular video trace. Display systems in which this is not done are especially difficult to use because there is inadequate CRT screen height to show large signals added to the distance amplitude compensation waveshape curve.
Various prior art devices use a flaw gate which produces an output of alarm when the echo amplitude in the gate portion of the time sweep rises above a selected level, as shown, for example, in U.S. pat. No. 2,883,860 to Henry. Such conventional devices use a rectangular gating function having a length corresponding to the material depth to be examined and a constant amplitude corresponding to the alarm level. The gating function of such devices has essentially constant sensitivity along its length and, to achieve automatic distance amplitude compensation, the receiver amplifier gain in such devices is controlled by distance amplitude compensation techniques as described above with the inherent pitfalls as set forth above.